1. Field of the Invention
The present invention relates to a high pressure discharge lamp and a method for sealing a bulb of the high pressure discharge lamp. More specifically, the present invention relates to a high pressure discharge lamp which may be used as a light source for such devices as a copier or a projector and which, even after being lit for a considerably long time, does not have problems such as a blowout of the bulb made of quartz glass or the blackening of the quartz glass bulb, and a method for sealing a bulb used in such a high pressure discharge lamp.
2. Description of the Related Art
In general, high pressure discharge bulbs have a structure in which each electrode of a pair of electrodes (i.e., an anode and a cathode) is disposed so as to be opposite the other in a quartz glass bulb, which includes an expanded portion for luminescence and a sealing portion, and the anode and the cathode are joined by, for instance, welding with molybdenum foil. Also, the sealing portion of the quartz glass bulb is airtightly sealed by, for example, welding with molybdenum foil. A gas for assisting an electric discharge, such as mercury vapor, is contained in the expanded portion for luminescence of the quartz glass bulb which has been airtightly sealed.
In general, high pressure discharge lamps which may be represented by a xenon lamp, a high pressure mercury lamp, and a metal halide lamp, have high brightness and excellent color rendering properties, and are used as light sources for such devices as a copier or a projector.
FIG. 8 is a diagram showing a configuration of a conventional high pressure discharge lamp. The high pressure discharge lamp includes a pair of electrodes 2a and 2b, which are disposed so as to be opposite each other, and a material which is capable of maintaining a light discharge of the lamp is contained in a tubular quartz glass bulb 1. The electrodes 2a and 2b are connected to molybdenum (Mo) foils 3a and 3b, respectively, and both ends of the quartz glass bulb 1 are sealed with a part of the electrodes 2a and 2b and the Mo foils 3a and 3b. The electrode 2a and the Mo foil 3a, and the electrode 2b and the Mo foil 3b, respectively, may be connected by using a welding method. Also, mercury vapor and an inert gas, for instance, are used as the materials capable of maintaining a light discharge and are contained in the quartz glass bulb 1.
In the above-mentioned high pressure discharge lamp, an external lead wire (not shown in the figure) is connected to the Mo foils 3a and 3b, which are sealed at their respective ends of the quartz glass bulb 1, and a predetermined trigger voltage is applied to the external lead wires. When the trigger voltage is applied, a glow discharge is induced between the electrodes 2a and 2b in the inert gas atmosphere thereby vaporizing mercury contained in the quartz glass bulb 1, and this causes a plasma discharge in the high pressure mercury vapor. The light emitted by the plasma discharge has high brightness and excellent color rendering properties.
As mentioned above, in the high pressure discharge lamp, an inert gas is contained and sealed in the quartz glass bulb 1 as a starting gas for the glow discharge, and the charged pressure thereof is between 6 kPa and 60 kPa (preferably between 20 kPa and 50 kPa). For this reason, the difference in pressure between the inside and outside of the quartz glass bulb 1, i.e., the difference between the atmospheric pressure and the charged pressure, is in the range between 41 kPa and 95 kPa. As a method for carrying out airtight-sealing of the quartz glass bulb 1, a pinch sealing method and a shrink sealing method are known.
The pinch sealing method is a method in which an outer periphery portion of a quartz glass bulb is collapsed and sealed by pressure applied using a force piston provided with a metallic mold. This method is mainly used for a sealing process in which the internal pressure of an object is about 4–5 MPa. If the pinch sealing method is employed, however, residual distortion tends to be generated after applying pressure. Also, stress concentrations tend to be caused since the shape of the quartz glass bulb and that of a sealing metal at the contacting portion thereof are significantly different. Accordingly, if the pinch sealing method is applied to the above-mentioned high pressure discharge lamp, there is a danger that the quartz glass bulb 1 may blow out.
The shrink sealing method, on the other hand, is a method in which an outer periphery of both ends of a quartz glass bulb is heated while the pressure difference between the inside and outside of the quartz glass is maintained, and, thereafter, the quartz glass bulb is naturally shrunk in order to airtightly seal the bulb. The shrink sealing method is applicable to a sealing process in which the internal pressure of a quartz glass bulb is 20 MPa or greater. According to this method, contrary to the pinch sealing method, a forced pressure is not applied to the bulb and residual distortion does not tend to be generated since the quartz glass bulb is subjected to natural shrinkage. Also, since the shape of the quartz glass bulb and that of a sealing metal foil are substantially the same, stress concentration tends not to be caused. For this reason, the shrink sealing method is often used as the sealing process for the above-mentioned high pressure discharge lamp.
However, in the conventional shrink sealing method, the difference between the thermal expansion coefficient of the quartz glass bulb and that of the electrodes is not considered at all when carrying out the sealing process of the bulb of a high pressure discharge lamp. Also, a heating step for the quartz glass bulb during the sealing process is conventionally carried out manually and it was difficult to obtain an accurate target length of a contacting portion which is formed by contacting an electrode with the quartz glass bulb at the sealing portion. For the case where the length of the contacting portion formed by the electrode and the quartz glass bulb at the contacting portion is relatively long, cracks may be generated at the sealing portions, as shown in FIG. 9, due to the difference between the thermal expansion coefficient of the electrode and the quartz glass bulb. When the internal pressure of the quartz glass bulb 1 is increased upon lighting the high pressure discharge lamp, the cracks may develop into a large cleft and become the cause of a bulb blowout. Moreover, although it is possible to suppress the generation of cracks by decreasing the length of the contacting portion formed by the electrode and the quartz glass bulb at the sealing portions, there is a danger that problems such as the falling of an electrode may be caused by decreasing the length of the contacting portion.
On the other hand, when a conventional high pressure discharge lamp is used, sputtering is vigorously caused and this causes blackening of the quartz glass bulb in a relatively short amount of time. Also, if the amount of halogen gas contained in the high pressure discharge lamp is increased to enhance the halogen cycle efficiency in order to prevent the blackening caused by the electrode sputtering, the sealing portion of the electrode tends to be eroded by the halogen gas and this eventually causes a blowout of the quartz glass bulb.
Accordingly, an object of the present invention is to solve the above-mentioned problems and provide a high pressure discharge lamp and a method for sealing a bulb thereof by which the generation of cracks during the sealing process may be suppressed and problems such as the falling of an electrode are not caused.
Another object of the present invention is to provide a high pressure discharge lamp by which a blowout of the quartz glass bulb or the blackening of the quartz glass bulb may be prevented even after being lit for a considerably long time.
The inventors of the present invention, after pursuing diligent studies to achieve the above-mentioned objectives, have noticed the importance of the length L of the contacting portion which is formed by contacting the electrode and the quartz glass bulb at the sealing portion be in the range between Lmax (mm)≦200/(P×D) (the maximum length) and Lmin (mm)≧0.8/(D2×π) or Lmin (mm)≧0.7 whichever is longer (the minimum length), where D is the diameter (mm) of the electrode and P is the power (W) supplied to the high pressure discharge lamp.
Also, the inventors of the present invention have noticed the importance in the roughness of the surface of an end portion of the electrode and discovered that if the maximum value (hereinafter referred to as “Rmax”) of the surface roughness (hereinafter referred to as “R”) of the end portion of the electrode is less than a certain value, it becomes possible to significantly decrease the sputtering of the electrode and, hence, prevent the blackening of the quartz glass bulb. The inventors of the present invention have also discovered that if the Rmax value of portions of the electrode other than the end portion is within in a certain range, it becomes possible to prevent a blowout of the quartz glass bulb.